Complementary metal-oxide-semiconductor (CMOS) technology has continued to evolve for the past 40 years with integrated circuit (IC) density quadrupling every three years to reduce cost and improve device performance (e.g. speed). Denser circuits need device scaling with requirement of increasingly compact (shallow and rapidly varying) doping profiles in both the source/drain junctions. The International Technology Roadmap for Semiconductors (ITRS) anticipates junctions around 25 nanometers (nm) deep, with sheet resistance of 250 Ohm/square for the 100 nm lithography technology generation. Junctions this shallow have approached the process limits of conventional implantation and thermal annealing processes. It is well known that boron dopants experience transient enhanced diffusion (TED) during post-implant annealing due to their interaction with the Si interstitials and that the activation of dopants is limited by the thermal solid solubility, as shown by A. E. Michel, W. Rausch, P. A. Ronsheim, and R. H. Kastl, Appl. Phys. Lett., 50 (1987) 416, and P. A. Stolk, H-J, Gosssmann, D. J. Eaglesham, D. C. Jacobson, C. S. Rafferty, G. H. Gilmer, M. Jaraiz, J. M. Poate, H. S. Luftman, and T. E. Haynes, J. Appl. Phys. 81, 6031 (1997). It has become a daunting task to meet ITRS requirement with traditional methods such as very low energy ion implantation followed by rapid thermal annealing.
Furthermore, the conventional ion implantation techniques used to dope submicron silicon devices have throughput problems attributed to the fact that an ion beam implanter current drops significantly due to space charge limitations at very low ion energy. The attempts to meet these major technology challenges have led to the development of new processes which include: 1) ultra low energy implantation with energy reduced to sub keV region; 2) cluster ion implantation; 3) other doping methods such as plasma immersion ion implantation; 4) low thermal budget process with spike rapid thermal annealing and 5) other annealing processes such as laser annealing.
Each of these methods has its advantages and disadvantages. However, all of them have the disadvantage that they cannot solve the most serious problem, the spreading of the dopant profiles due to the transient enhanced diffusion and boride-enhanced diffusion (BED). For example, even though the very low energy boron implantation technique produces a shallow implantation profile, a burst of TED is observed upon annealing resulting in a junction several hundred nanometers (nm) deep. This phenomenon, known as transient enhanced diffusion, is caused by the interaction with silicon interstitials with the boron ions which are either on lattice sites in crystal silicon or in interstitial sites. Transient enhanced diffusion is observed for dopants that are mobile in Si through an interstitialcy mechanism, and have diffusivity enhancement proportional to the enhanced interstitial concentration. As a consequence of the implant damage due to collision cascades, the dopant diffusivity enhancement may be 102-104 times larger than equilibrium diffusivity values. Another anomalous diffusion phenomenon, called boride-enhanced diffusion, has been observed in the proximity of a silicon layer containing a high boron concentration. (See, for example, Aditya Agarwal, H.-J. Gossmann, D. J. Eaglesham, S. B. Herner, A. T. Fiory, and T. E. Haynes, Appl. Phys. Lett 74, 2435 (1999)). Boride-enhanced diffusion is driven by the Si interstitials injected from the silicon boride layer during annealing. BED is a greater problem than TED because high concentrations of boron are necessary in any doping method used. Therefore there is a need in the art for methods for forming ultra-shallow junctions.
Several prior art approaches have attempted to reduce the transient enhanced diffusion for shallow junction formation. In one approach, a carbon co-implant was used to reduce the transient diffusion of boron dopant during rapid thermal anneal (RTA). The conditions employed in forming the shallow junction using carbon co-implantation were reported by P. A. Stolk, H-J, Gosssmann, D. J. Eaglesham, D. C. Jacobson, C. S. Rafferty, G. H. Gilmer, M. Jaraiz, J. M. Poate, H. S. Luftman, and T. E. Haynes, in J. Appl. Phys. 81, 6031 (1997) as follows: 30 keV B implant, dose 1.5×1014/cm2, multiple carbon implant to a level of 4-6×1018/cm3, and regrown at 500° C. for 1 h, 600° C. for 2 h, and 900° C. for 15 min. Although carbon co-implant is effective in reducing the transient diffusion of boron, this method suffers from the disadvantage that high density of residual defects remain after RTA. This is the case even using high temperature anneal conditions. The high density of residual defects results in high electrical leakages for the shallow junction.
Another approach reported by T. H. Huang et al. (“Influence of Fluorine Preamorphization on the Diffusion and Activation of Low-energy Implanted Boron during Rapid Thermal Anneal,” Appl. Phys. Lett., (1994) Vol. 65, No. 14, p. 1829) used fluorine co-implants to reduce the transient diffusion of boron dopants during rapid thermal anneals. The conditions used in this reference for shallow junction formation are as follows: fluorine implant, 40 keV ion energy, dose 2×1015/cm2, 5 keV boron or 23 keV BF.sub.2 shallow implants. In the process disclosed by Huang et al., the wafers were rapid thermal annealed at 1000° C., 1050° C. and 1100° C. for 30 seconds. Although the presence of fluorine implants reduced the transient boron enhanced diffusion during RTA, this prior art method also suffers from the disadvantage that residual defects remain after 1000° C., 30 seconds anneal. Residual defects can only be removed with 1100° C., 30 seconds anneal. However, substantial dopant motion occurs at this higher temperature and therefor ultra-shallow junctions cannot be formed.
Another approach reported by S. Saito entitled “Defect Reduction by MeV Ion Implantation for Shallow Junction Formation,” Appl. Phys. Lett., (1993) Vol. 63, No. 2, p. 197 used fluorine implants for preamphorization (40 keV, 1015/cm2), shallow implant; boron at 10 keV and 5×1015/cm2. This was followed by ion implantation of either fluorine or silicon at 1 MeV energy or arsenic at 2 MeV energy. The dose used for the MeV implant was between 5×1014/cm2 and 5×1015/cm2 The samples were rapid thermal annealed at 1000° C. or 1100° C. for 110 seconds. Under these experimental conditions, Saito demonstrated that the MeV implants were effective in reducing the boron transient diffusion with and without fluorine preamphorization. This reference also demonstrated that maximum reduction in boron dopant diffusion was achieved when both fluorine preamorphization and Mev fluorine implants were used. However, as mentioned in the prior art earlier, use of fluorine implants creates residual defects and requires temperatures as high as 1100° C. for low leakage junction to be formed.
In U.S. Pat. No. 6,037,640, Lee proposes a combination of low energy shallow dopant implant combined with high energy deep ion implant. Lee proposed that the high energy implant causes deeply buried defects which act as gettering centers for the interstitials and which are stable, and does not mention or suggest an excess of vacancies near the surface. Lee, however, showed junction depths of greater than 20 nm for the preamorphized and boron implanted samples, and junction depths greater than 30 nm if no preamorphization was used. In all cases, the dose of implanted boron used by Lee was sufficient to amorphize the surface of the silicon when a sufficient dose of boron for low sheet resistance was used.
The prior art shows no doping method where boron is introduced on to or into a silicon surface which remains crystalline, and which has an excess of vacancies to act to reduce the TED and BED of the boron, where the annealed junction depth is less than or equal to 20 nm and where the doping level when annealed is sufficient to provide a sheet resistance of less than 400 ohms/square.
A method for forming ultra shallow junctions that overcome the disadvantages noted above is provided herein.